Among electroluminescence elements, an organic electroluminescence element (hereinafter, sometimes referred to as “organic EL element”) using an organic substance has recently been regarded as a promising element as a solid luminescence-type inexpensive large-area full-color display element or a writing light source array, and therefore has been actively researched and developed.
An organic EL element is a thin film-type all-solid element including a film, a pair of positive and negative electrodes formed on the film, and a functional layer (single layer part or multi-layered part) that is sandwiched between the electrodes, contains an organic light-emitting substance, and has a thickness of only about 0.1 μm. When a relatively low voltage of about 2 to 20 V is applied to such an organic EL device, electrons and holes are injected into the organic compound layer from the negative electrode and the positive electrode, respectively. It is known that these electrons and holes recombine in the light-emitting layer, and energy is emitted as light when the energy level returns from the conduction band to the valence band so that luminescence occurs. This technique is expected to be applied to next-generation flat displays or lighting devices.
As one of measures for preventing global warming, the application of the technique to lighting devices that account for a large proportion of energy consumption for people has begun to be examined. Attempts have been actively made to improve the performance and reduce the cost of a white light-emitting panel, which can be an alternative to conventional lighting devices, for its practical use.
A white light-emitting panel for lighting is required to have high efficiency and long service life. In fact, however, a white light-emitting panel using an organic EL device is inferior to a fluorescent light or a white-light LED in terms of an increase in service life.
As a method for solving such a problem, there is a method using an inorganic light-emitting substance, “quantum dot material” as a light-emitting material.
A quantum dot material has a sharp emission spectrum, is an inorganic material and therefore excellent in durability, and is soluble in various solvents and therefore applicable to a coating process.
For example, Patent Literature 1 discloses an electroluminescence element having a light-emitting layer containing a quantum dot material.
Further, Patent Literature 2 discloses an organic electroluminescence element in which a light-emitting layer contains a phosphorescence-emitting dopant and the light-emitting layer or a layer adjacent to the light-emitting layer is doped with a quantum dot material.